1. Field of the Invention
The present invention relates to a front photo detector (FPD) for an optical pick-up, and more particularly, to an FPD for an optical pick-up that does not need a gain selection switch and thus does not need to control gain selection and prevents an abnormal operation due to gain selection, thereby having improved reliability.
2. Description of the Related Art
A laser diode is used as a light source for an optical pick-up of an optical recording and playing apparatus such as a compact disc (CD) player, a CD-Recordable drive, or a CD-ReWritable drive. To ensure the smooth operation of the optical recording and playing apparatus, the power of the laser diode should be stabilized; however, this is difficult because the power of the laser diode changes significantly with temperature and period of use. To maintain the power of the laser diode at a proper level, power control is required; therefore a laser diode power control device is included in the optical pick-up.
The laser diode power control device in the optical pick-up of the optical recording and/or playing apparatus includes an FPD and an automatic power control (APC) circuit. The FPD receives a portion of the light emitted from the laser diode of the optical pick-up by using a photo diode, converts current that is generated from the photo diode in proportion to the power of the received light into a voltage by using a current-voltage amplifying circuit, amplifies the converted voltage by using a voltage amplifying circuit, and outputs the amplified voltage to the APC circuit. The FPD is generally used to obtain a monitoring voltage for recording power control.
The APC circuit receives an output voltage of the FPD, compensates for a difference between the output voltage and a predetermined reference voltage, and outputs a proper laser diode driving current.
FIG. 1 is a circuit diagram of a conventional FPD.
Referring to FIG. 1, an FPD 100 includes a photo diode 101, a current-voltage amplifying circuit 110, and a voltage amplifying circuit 120. The photo diode 101 receives a portion of a light emitted for recording from a laser diode and generates a current that is proportional to the power of the received light. The current-voltage amplifying circuit 110 converts the current generated by the photo diode 101 into a voltage. The voltage amplifying circuit 120 amplifies the input voltage from the current-voltage amplifying circuit 110 by a predetermined gain.
Vref denotes a reference voltage input to amplifiers 125 of the current-voltage amplifying circuit 110 and the voltage amplifying circuit 120.
A gain of the voltage amplifying circuit 120 can be adjusted by adjusting input resistances that are adjusted by adjusting a variable resistor VR disposed at an input terminal of the voltage amplifying circuit 120. The variable resistor VR is adjusted before being released into the market. Such adjustment is designed to output the same voltage to an APC circuit when the same light power is input to the FPD 100.
FIG. 2 is a circuit diagram of a conventional FPD of an optical pick-up in which two laser diodes are used.
FIG. 1 shows the FPD 100 of an optical pick-up that uses a single laser diode, e.g., an optical pick-up that only performs recording/reproducing with respect to CDs. In contrast, FIG. 2 shows an FPD 200 of an optical pick-up in which two laser diodes that emit lights of different wavelengths are selectively used to perform recording/reproducing with respect to CDs and DVDs.
As shown in FIG. 2, the FPD 200 includes a photo diode 201, a current-voltage amplifying circuit 210, and a voltage amplifying circuit 220. The photo diode 201 receives a portion of light from a selected laser diode that emits light for recording and generates current that is proportional to the power of the light emitted from the selected laser diode. The current-voltage amplifying circuit 210 converts the current generated by the photo diode 201 into a voltage. The voltage amplifying circuit 220 amplifies the voltage input from the current-voltage amplifying circuit 210 by a predetermined gain.
Unlike the current-voltage amplifying circuit 110 of FIG. 1, a current-voltage amplifying circuit 210 of FIG. 2 includes two feedback resistors R1 and R2 that can be selectively used. Also, unlike the voltage amplifying circuit 120 of FIG. 1, a voltage amplifying circuit 220 of FIG. 2 includes two input variable resistors VR1 and VR2 that can be selectively used.
Selecting one of the two feedback resistors R1 and R2 or one of the two variable resistors VR1 and VR2 is performed by a gain selection switch 230. The gain selection switch 230 operates with a selection signal input from a control unit of the optical recording and playing apparatus. The selection signal is output from the control unit to the gain selection switch 230 after the control unit identifies the type of a currently used recording medium, so that a gain corresponding to the identified recording medium can be selected.
In FIG. 2, the feedback resistor R2 and the input variable resistor VR2 are selected. Alternatively, the feedback resistor R1 and the input variable resistor VR1 may be selected.
The wavelength of the light emitted from a laser diode for CDs is approximately 650 nm, while the wavelength of the light emitted from a laser diode for DVDs is approximately 780 nm. Also, the power of the emitted light for the laser diode for CDs is different from the laser diode for DVDs. Thus, the sensitivity of a conventional FPD 200 should be changed according to whether the laser diode for CDs or the laser diode for DVDs is being used. That is, the voltage output from FPD 200 should be different even if the same power is input. Therefore, during product manufacturing, an input variable resistor for CDs and an input variable resistor for DVDs are separately adjusted to respectively control gains depending on whether the laser diode for CDs is used or the laser diode for DVDs is used.
As shown in FIG. 2, the conventional FPD 200 necessarily includes the gain selection switch 230 and inputs the selection signal output from the control unit to the gain selection switch 230 after the type of the currently used recording medium is identified; thereby, selectively using one of the feedback resistors R1 and R2 and one of the input variable resistors VR1 and VR2 that are suitable for the currently used recording medium.
Thus, the gain selection switch 230 has to be included in the conventional FPD 200 and a separate signal line has to be connected to the gain selection switch 230 from the control unit, resulting in a complicated configuration. Further, due to possible noise in the selection signal, there may be an abnormal operation in gain selection.